The present invention relates generally to systems for the transmission and reception of multi-carrier, high speed data signals. More particularly, a discrete multi-tone (DMT) system having a widened bandwidth is described.
At the time of this writing, the Alliance For Telecommunications Information Solutions (ATIS), which is group accredited by the ANSI (American National Standard Institute) Standard Group, is nearing finalization of a standard for the transmission of digital data over Asymmetric Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting video data over ordinary telephone lines, although it may be used in a variety of other applications as well. The standard is based on a discrete multi-tone transmission system. The pending North American Standard is referred to as the T1E1.4 ATIS Standard, and is presently set forth in Standard Contribution No. 94-007, rev. 1, dated January of 1994, which is incorporated herein in its entirety. Transmission rates are intended to facilitate the transmission of information at rates of at least 6 million bits per second (i.e., 6+Mbps) over ordinary phones lines, including twisted-pair phone lines. The standardized discrete multi-tone (DMT) system uses 256 "tones" that are each 4.3125 kHz wide in the forward (downstream) direction. That is, in the context of a phone system, from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user).
The Asymmetric Digital Subscriber Lines standard also contemplates the use of a duplexed reverse signal at a data rate of at least 608 Kbps. That is, transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term Asymmetric Digital Subscriber Line comes from the fact that the data transmission rate is substantially higher in the forward direction than in the reverse direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over the telephone lines. By way of example, one potential use for the systems allows residential customers to obtain videos information such as movies over the telephone lines rather than having to rent video cassettes. Another potential use is in video conferencing.
As is well known to those familiar with the Asymmetric Digital Subscriber Lines standardization process, as well as telephone systems generally, most telephone systems are divided into a multiplicity of carrier service areas that have a desired maximum carrier service area (CSA) range of 2 miles from a "central office" when 24-gauge twisted pair wiring is used and 9000 feet when 26-gauge wiring is used. Thus, one of the important features in the standardization process was that the selected system be capable of being transmitted throughout a CSA range from a central office over ordinary 24 gauge twisted-pair phone lines. This requires both that the signal does not attenuate an unreasonably high amount and that it be relatively tolerant of crosstalk noise.
One acknowledged drawback of the discrete multi-tone solution for the Asymmetric Digital Subscriber Line standard is that when T1 crosstalk noise is present in the same binder or an adjacent binder, the transmission scheme has difficulty reaching the outer limits of the prescribed range with reliable signals. T1 circuits generally carry 24 voice channels at a data rate of approximately 1.544 million bits per second and are generally known to create a significant amount of crosstalk noise. In fact, the presence of T1 noise will generally cut the range of a digital multi-tone signal to less than the CSA range given the desired power limitations and permissible bit error rates. Accordingly, it appears that special provisioning may be required to ensure complete carrier servicing area coverage in all cases. Although the types of telephone systems that experience T1 noise are quite low on a percentage basis, it is widely perceived as being important to have 100 percent compatibility in any standardized service. It would, of course, have been desirable to guarantee complete carrier service area coverage with a standardized technology. However, it was widely believed that such a range was impossible given the nature of the discrete multi-tone technology. The present invention is believed to be one solution to the T1 crosstalk noise problem. The present invention also provides a solution that has numerous advantages well beyond the mitigation of T1 noise problems. Specifically, it is equally applicable to phone systems that experience E1 noise (which are primarily located outside of North America), although the problem is more pronounced in areas that experience T1 crosstalk noise. Further, in areas which are not susceptible to T1 or E1 crosstalk noise (which is the vast majority of the installed telephone system base), the described invention permits reliable transmission of digital information at the rate of 15 Mbps per second at distances of at least a mile. The system also permits a provision for higher speed transmissions in the upstream direction.